Balanced ultra high frequency generator



Dec. 2, 1947. L, R, MA| |NG I I 2,431,762

BALANCED ULTRA-HIGH-FREQUENGY GENERATOR Filed March 23, 1944 3 0 Lin-ligo Symmmry Fl G@ Fewer Output INVENTOR LEV!) R. my@ BY Patented Dc. 2,1947 UNITED STATES PATENT OFFICE BALANCED ULTRA mon FREQUENCY GENERATORALeonard R. Malling, San Diego. Calif., assigner, by mesne assignments,to Hazeltine Research, I nc., Chicago, Ill., a. corporation of IllinoisApplication March 23, 1944, Serial No. 527,753

11 Claims.

This invention relates to balanced ultrahigh-frequency oscillationgenerator tunable over tuning the generator over a prescribed range ofoperating frequencies. A source of space current is applied to the anodeelectrode of the oscillator tube through one oi the conductors oi theline, being isolated from the controll electrode thereof by means of asuitable blocking condenser which is generally incorporated in theshort-circuiting member.

The operating frequency range of such a generator is determined by theallowable travel of the short-circuiting member, the highest obtainablefrequency being realized when this member is advanced along the line astar as possible in the direction of the oscillator tube. The value ofthis highest obtainable frequency is determined by the distributedparameters of the line and the anode-control electrode capacitance ofthe oscillator tube which constitutes a loading effect at the electrodeterminals of the line. Where a wide operating frequency range isdesired, the oscillation generator may also be provided with a secondeffectively resonant transmission line which is coupled to theoscillator l tube and tunable "in the manner described. In

this case, the lines are tuned together and an increased range ofoperating frequencies results since the loading eiiect for each linecontributed by the interelectrode capacitance ofthe oscillator tube isapproximately one-half that of the singleline arrangement. Suchoscillation generators,

since they include a single oscillator tube, are

capable of delivering but a limited amount of power. Additionally, it isdifoult to construct blockingl condensers which have suitable electricalcharacteristics at ultra-high frequencies and can electrodes and asecond effectively .resonant transmission line coupled between theircontrol electrodes. members for tuning which are 'such that thegenerator is of the tuned-plate tuned-grid type. While thesearrangements provide an increased power output as compared with that ofthe single-tube oscillation generators mentioned aboveI they have alimited range of operating frequencies due to the loading effectcontributed to the electrode-terxninal portions of the transmissionlines by the interelectrode.capacitance of the oscillator tubes.Furthermore, in view of the tuning difficulties presented, it has notbeen considered expedient to double the line arrangement in order toreduce this loading effect, thereby to increase the operating frequencyrange of such generators.

It is an object of the invention, therefore, to provide an improvedbalanced ultra-high-frequency oscillation generator which is tunableover a predetermined range of operating frequencies and which avoids oneor more of the above-mentioned limitations of prior art arrangements.

It is another object of the invention to provide an improved balancedultra=high-frequency oscillation generator which is tuiiable over arelatively wide range of operating frequencies. y

It isistill a further object of the invention tio provide a balancedultra-high-frequencw oscillation generator tunable over a predeterminedrange of operating frequencies and having an iinproved stabilitycharacteristic.

In accordance with the invention, a balanced ultra-high-frequencyoscillation generator tunable over a predetermined range of operatingfrequencies comprises a pair of vacuum tubes individually having anodeand control electrodes. The generator includes a i'lrstv balancedtransmis? sion line comprising a pair of open conductors individuallydirectly coupled to corresponding electrodes of the vacuum tubes andincluding adjustable tuning means which constitute therewith i afrequency-adjusting circuit for tuning the generator over theaforesaidrange of operating frequencies. The generator further includesa second balanced transmission line4 comprising a pair of conductorsindividually directly coupled to the other corresponding electrodes ofthe vacuum tubes and extending therefrom in the same direction as therst-mentioned line. The second line has a resonant frequency outside ofthe range of operating frequencies of the generator and is electricallycoupled tothe first lineI for substantially all adjustments oi thetuning means The lines include short-circuitingV for transferring energyfrom the first to the second line to sustain oscillations.

'In the specific application of the, invention to be describedhereinafter, a balanced ultra-highfrequency oscillation generatorincluding a frequency-determining circuit and a pair of vacuum tubesindividually having anode, cathode and control electrodes comprises apair of short-circuited transmission lines. Each of the lines has aneffective electrical length substantially equal to an integral multipleof a quarter-wave length at a predetermined operating frequency of thegenerator and each line comprises a pair of open conductors individuallycoupled to corresponding electrodes of the vacuum tubes. Thefrequencydetermining circuit has at the aforementioned operatingfrequency a desired mode of resonance in which the currents invcorresponding portions of the conductors of each of the lines are inphase opposition and the lines are included in an additional circuitwhichhas an undesired Inode of resonance in which the currents incorresponding portions of the conductors are in phase. The generatorincludes means for so loading the lastnamed circuit that the undesiredmode of resonance occurs at a predetermined frequency substantially.different from the aforementioned operating frequency of the generator.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in connection .with the accompanying drawing, and itsscope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a schematic circuit diagram of a balancedultra-high-frequency oscillation generator embodying the presentinvention; Fig. 2 is a schematic circuit diagram which is electricallyequivalent to the circuit of Fig. 1; Figs. 3a and 3b are additionalschematic circuit diagrams utilized in describing an operatingcharacteristic of the Fig. 1 arrangement; while Fig. 4 comprises a graphrepresenting the power output-frequency characteristic of theoscillation generator represented in Fig. 1.

Referring now more particularly' to Fig. 1 of the drawing, there isrepresented a balanced ultra-high-frequency oscillation generatortunable over a prescribed wide range of operating frequencies andembodying the present invention. The generator includes a pair of vacuumtubes I and `II individually having anode, cathode and controlelectrodes. The generator also includes a first plurality of balancedtransmission lines individually directly coupled to correspondingelectrodes of each of the vacuum tubes. Specifically, one transmissionline vcomprising a pair of open parallel conductors I2, I3 is directlycoupled to the anode electrodes of tubes I0 and II, respectively, whileanother transmission line comprising a pair of open parallel conductorsI4, I5 is similarly coupled to the anode electrodes of the vacuum tubes.The conductors of each such line have uniformly distributed inductanceand capacitance and the lines are symmetrically disposed with referenceto the vacuum tubes, extending therefrom in opposite directions, asrepresented in the drawing. This relationship of lines I2, I3 and I4, I5may be described as a feed-through arrangement since conductors I4 AandI5 may be considered effectively to comprise extensions of conductors I2and I3, respectively. Adjustable short-circuiting elements .I6 and I1are included, respectively, in lines I2, I3 and I4. I5 andare arrangedto frequencies.

The generator further includes a second plurality of balancedtransmission lines corresponding in number with the first-describedplurality and individually directly coupled to other correspondingelectrodes of tubes I0, II. In particular, one transmission linecomprising a; pair of open conductors I8, I9 is directly coupled to thecontrol electrodes of the vacuum tubes, While another transmission linecomprising a pair of open conductors 20, 2l is similarly coupled to thecontrol electrodes of the tubes. The con ductors of lines I8, I9 and 20,2I have uniformly distributed inductance and capacitance and theirphysical arrangement with reference to tubes I0 and II is substantiallythe same as that described in connection with lines I2, I3 and I4, I5.In other Words, as represented in the drawing, corresponding ones of thedescribed first and second plurality of lines extend from the tubes inthe same direction. Each of lines I8, I9 and 20, 2I'is proportioned tohave an effective electrical length equal to an integral multiple of aquarter-wave length at a predetermined operating frequency outside of,and preferably above, the range of operating frequencies of thegenerator. That is to say, each such line has a resonant frequency whichlies above the range of operating frequencies of the generator. Thisproportioning of the electrical lengths of lines I8, I9 and ZIl, 2|, aswell as their physical relationship with reference to lines I2, I3 andI4, I5, is effective electrically to couple corresponding ones of v.I'lof line I4, I5.

the rst and second pluralities of lines for substantially alladjustments of the tuning means I 6, I'i for transferring energy fromthe first to the second plurality of lines in order to sustainoscillations. 'As an aid to this coupling, line I8, I9 includes aninverted or reversed loop portion 22 which is magnetically coupled toshort-circuiting element I8 of line I2, I3 for substantially alladjustments. of the short-circuiting element, while line 2li, 2Iincludes a reversed loop portion 23 magnetically coupled toshort-circuiting element These loop portions are inverted so that theenergy transferred between the lines has the proper polarity forsustaining oscillations.

The cathodes of tubes I0 and II are of the directly-heated type and havea pair of cathode leads which connect to a source of heater voltage, asindicated by the arrows. The mid-point of the cathode of tube I0 iscoupled to ground through a radio-frequency choke 30. Condensers 3| and32 are provided to maintain substantially the same radio-frequencypotential at both cath- A trol electrodes of the vacuum tubes aregrounded through a radio-frequency choke 4I and a grid resistor 42. anadjustable condenser 43 is coupled between the short-circuited terminalsof transmission line I4, I5 and ground for a purpose to be describedfully hereinafter. An output signal is obtained from the generator bymeans of a pick-up loop 5I) which is electrically coupled totransmission line I4, I5. This output signal may be supplied to asuitable utilizing circuit, such as an antenna system 5I, by way of acoaxial cable 52 which couples the antenna to pick-up loop 50. Theantenna circuit is tunable through an adjustable condenser 53.

In considering the operation of the described arrangement, it will beunderstood that shortcircuit-ing elements I E and i 'I are adjusted tohave substantially identical positions with reference to theirassociated transmission lines. For any given adjustment of theseshort-circuiting elements, transmission lines I2, I3 and I4, I5individually comprise an effectively resonant Vtransmission line havingan effective electrical length which is preferably equal to aquarter-wave length corresponding to a predetermined operating frequencyof the oscillation generator. As is well known from transmission-linetheory, any effectively resonant transmission line having such aneffective electrical length exhibits the impedance characteristics of aparallel resonant circuit. Therefore, each short-circuiting element withits associated transmission line provides a frequency-adjusting circuitfor the generator, the frequency-adjusting circuit provided by each suchline and its short-circuiting element having the same operating orresonant frequency. The particular value of this operating frequency isdetermined by: (1) the position of the short-circuiting element whichdetermines the effective .electrical length of the line; (2) thedistributed parameters of the line; and (3) the terminating impedance ofthe line which, for the case under consideration, comprisessubstantially one-haiil the interelectrode capacitance of the anode andcathode electrodes of vacuum tubes I and II since lines I2, I3 and I4,I5 are arranged in a parallel relationship with reference to theseelectrodes. Additionally, for any given adjustment of theshort-circuiting elements IB and I'I, transmission lines I8, I9 and 2U,2| have a resonant frequency which is above the operating frequency ofthe generator. For this reason, lines I3, I3 and 2li, 2l effectivelycomprise inductive reactances which are electrically coupled to linesI2, I3 and I4, I5 to provide the requisite feed-back voltage forsustaining oscillations. 'Ihe coupling between these lines utilized fortransferring feed-back lenergy is contributed largely by the loopportions 22 and 23 since these loop portions are magnetically coupledwith the short-circuiting elements of transmission lines I2, I3 and I4,I5 where there is a current maximum under stabilized oscillatingconditions.

From the foregoing discussion it will be evident that the schematiccircuit diagram of Fig. 2 is the electrical equivalent of the Fig. 1arrangement. In this equivalent circuit the distributed inductance oftransmission lines I2, I3-and I4, I5 is represented by the lumpedinductance L, while the distributed capacitance thereof and theanode-cathode interelectrode capacitance of tubes i0- and II arerepresented by the lumped capacitance C. Transmission lines I8, I 9 and20, 2|, which have been described as presenting an inductive reactancefor any given adjustment of the short-circuiting elements I6 and I1, arerepresented by the inductance L1 having a mutual coupling M with theinductance L. The circuit arrangement of Fig. 2 will be recognized as aconventional push-pull type oscillation generator, the operation ofwhich is well understood in the art so that a description thereof isunnecessary. For tuning the oscillation generator over its range ofoperating frequencies, the inductance and capacitance of itsfrequency-adjusting circuit are simultaneously adjustable, as indicatedin Fig. 2, through short-circuiting elements I6 and I'I which areeffective, respectively, to adjust the effective electrical lengths oftransmission lines I2, I3 and I4, I5 to integral multiples of aquarter-wave length corresponding to a desired operating frequency.

Willie a balanced oscillation generator of the type under considerationhas satisfactory stability characteristics when constructed of lumpedparameters, an unstable operating condition is possible when thegenerator employs transmission lines arranged in a feed-through manner,as represented in Fig. 1'. This unstable operating condition may beattributed to a parasitic mode of resonance of the transmission lineswhich comprise the frequency-adjusting circultof the gcnerator and willbe described with particular reference to the sketches of Figs. 3a and3b.

The diagram of. Fig. 3a represents the frequency-adjusting circuit ofthe generator provided by the short-circuited effectively resonanttransmission lines I2, I3 and I4, I5 for a particular operatingcondition. This circuit is repre-"" sented as having, at the particularoperating frequency of the generator, a desired mode of resonance inwhich voltage r. *.xima occur at the anode electrodes. as represented bythe polarity indications, and in which current maxima occur at theshort-circuited terminals of each transmission line. The currentdistribution along the lines is such that the current in correspondingportions of the conductors of each line is in phase opposition. Thiscurrent relationship is indicated by the arrows. In this, the desiredmode of resonance, the frequency-adjusting circuit has an axis ofelectrical symmetry which corresponds to that of the generator and theresonant frequency varies with adjustments of short-circuiting elementsI6 and I'I.

The condition represented in Fig. 3b is an unde-.

sponding portions of the conductors of each line are in phase. For thismode of resonance the circuit is symmetrical about an axis which ispei'- pendicular to the axis of electrical symmetry of the generator.When this undesired mode of resonance occurs, conductors I2 and I4 areeffectively in parallel with conductors I3 and I5 in a circuit which iscompleted through ground by way of the condensers C', shown inbroken-line construction since they comprise the inherent capacitance toground at the short-circuited terminals oi.' the transmission lines. Itmay be shown to a rst .approximation that the frequency of thisundesiredmode of resonance is independent of the position ofshort-circuiting elements I6 and I 1.

Where the desired and the undesired mode of resonance appear at the sameoperating frequency of the generator, the power output is seriouslyreduced because the circuit exhibiting the unde-- sired mode ofresonance acts as a load on the generator and absorbs a large portion ofthe power. In accordance with the invention, condenser 43 is coupled totransmission line I4, I5

' as well as the described geometrical relationship for the purpose ofso loading the circuit arrangement represented by the diagram of Fig.. bthat the undesired mode of resonance occurs at a predetermined frequencywhich is substantially different from any desired operating frequency ofthe generator. The frequency-adjusting circuit has a voltage minima atthe short-circuited terminals of the transmission lines so that couplingthe loading condenser 43 at this point has substantially no effect onthe operating frequency of the frequency-adjusting circuit.

The power output-frequency characteristic of the oscillation generatorof Fig. 1 is represented by the full-line curve of Fig. 4. In thisfigure, the frequency designations f1 and f2 represent the lower andupper limits, respectively, of the desired operating frequency range ofthe oscillation generator. Over this range the power output has asubstantially constant value. The loading effect of condenser 43 causesthe reduced power output, resulting from the aforedescribed undesiredmode resonance, to occur at a frequency f4 outside of the desired range.In absence of this loading feature, a reduced power output may occur atsome frequency fa within the desired operating frequency range.

A balanced ultra-high frequency oscillation generator in accordance withthe present invention will be seen to have a relatively wide range ofoperating frequencies by virtue of the feedthrough line arrangementutilized as the frequency-'adjusting circuit. Furthermore, thearrangement is characterized by an improved n stability and produces asubstantially constant power output at all operating frequencies within4the desired Wide range.

Where an unusually wide operating range is not required, it will beapparent that a single balanced transmission linemay be coupled to theanode electrodes of tubes I0 and and a single balanced transmission linemay likewise be coupled to the control electrodes thereof. In any case,the transmission lines may have any of a variety of shapes. It is onlynecessary that the lines be so shaped and arranged that there is anelectrical coupling between the lines connected to the anode electrodesand those connected to the control electrodes of the vacuum tubes forthe purpose of providing a voltage feed-back path.

Oscillation generators constructed in accordance with the invention andarranged for operation at very high frequencies may include transmissionlines of relatively short physical lengths. In particular, lines I8, I9and 20, 2| have progressively shorter physical lengths for higheroperating frequencies of the generator. In the extreme case, these lineshave the physical appearance of a single-turn inductor and, in such a.case, the inductor leads may be considered to comprise the openconductors of a transmission line. Furthermore, while lines I8, I9 and20, 2| include In this alternative form of the invention, the

described effective electrical length of these lines with reference tothat of lines I2, I3 and I4, I5,

of such lines, aords sufiicient feedback to sustain oscillations.

It will be understood that the loading reactance, consisting ofcondenser 43, may be distributed and applied to the short-circuitedterminals of each of transmission lines I2, I3 and I4, I5 instead ofbeing coupled to a single line in the manner represented in Fig. 1.

While there have been described what are at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modications may be madetherein without departing from the invention, and itis, therefore, aimedin the appended claims to cover all such changes and modifications asfall within the true spirit and scope of the invention.

What is claimed is:

1. A balanced ultra-high-frequency oscillation generation tunable over apredetermined range of operating frequencies comprising, a pair ofvacuum tubes individually having anode and control electrodes, a firstbalanced transmission line' comprising a pair of open conductorsindividually directly coupled to correspondence electrodes of saidtubes, adjustable tuning means included in said line and constitutingtherewith a frequencyadjusting circuit for tuning said generator oversaid range of operating frequencies, and a second balanced transmissionline comprising a pair of conductors individually directly coupled tothe other corresponding electrodes of said tubes and extending therefromin the same direction as said first line, said second line having aresonant frequency outside of said range of operating frequencies andbeing electrically coupled to said first line for substantially alladjustments of said tuning means for transferring energy from said firstto said second line to sustain oscillations.

2. A balanced ultra-high-frequency oscillation generator tunable over apredetermined range of operating frequencies comprising, a pair ofvacuum tubes individually having anode and controlelectrodes, a firstbalanced transmission line comprising a pair of open parallel conductorsindividually directly coupled to corresponding electrodes of said tubes,adjustable tuning means included in said line and constituting therewitha frequency-adjusting circuit for tuning said generator over said rangeof operating frequencies, and a second balanced transmission linecomprising a pair of conductors individually directly coupled to theother corresponding electrodes of said tubes and extending therefrom inthe same direction as said first line, said second line having aresonant frequency outside of said range of operating frequencies andbeing electrically coupled to said rst line for substantially alladjustments of said tuning means for transferring energy from said firstto said second line to sustain oscillations.

3. A balanced ultra-high-frequency oscillation generator tunable over apredetermined range of operating frequencies comprising, a pair ofvacuum tubes individually having anode and control electrodes, a firstbalanced transmission line comprising a Pair of open conductorsindividually directly coupled to said anode electrodes of said tubes,adjustable tuning means included in said line and constituting therewitha frequencyadjusting circuit for tuning said generator over said rangeof operating frequencies, and av second balanced transmission linecomprising a pair of conductors individually directly coupled to saidcontrol electrodes of said tubes and extending therefrom in the samedirection as said first line, said second line having a resonantfrequency outside of said range of operating frequencies and beingelectrically coupled to said first line for substantially alladjustments of said tuning means for transferring energy from said firstto said second line to sustain oscillations.

4. A balanced ultra-high-frequency oscillation generator tunable over apredetermined fange of operating frequencies comprising, a pair ofvacuum tubes individually having anode and coni trol electrodes, a rstbalanced transmission line said range of operating frequencies, and asecond comprising a pair of open conductors individually directlycoupled to corresponding electrodes ,of said tubes, adjustableshortcircuiting means included in said line and constituting therewith afrequency-adjusting circuit fortuning said generator over said range ofoperating frequencies, and a'second balanced transmission line compris-eing a pair of conductors individually directly coupled to the othercorresponding electrodes of l said tubes and extending therefrom in thesame direction as said rst line, said second line having a resonantfrequency outside of said range of operating frequencies and beingelectrically coupled to saidrst line Afor substantially all adjustmentsof said short-circuiting means for transferring energy from said rst tosaid second line to sustain oscillations.

5. A balanced ultra-high-frequency oscillation generator tunable over apredetermined range of operating frequencies comprising, a pair ofvacuum tubes individually having anode and control electrodes, a firstbalanced transmission line comprising a pair of open conductorsindividually directly coupled to corresponding electrodes of said tubes,adjustable tuning means included in said line and constituting therewitha frequencyadjusting circuit for tuning said generator over balancedtransmission line comprising a pair of conductors individuallydirectlycoupled to the other corresponding electrodes of said tubes Aandextending therefrom in the same direction as vsaid first line, saidsecond line having an effective of operating frequencies comprising, apair of vacuum tubes individually having anode and control electrodes, afirst balanced transmission line comprising a pair of open conductorsindividually directly coupled to corresponding electrodes of said tubes,adjustable tuning means included in -said line and constitutingtherewith a frequencyadjusting circuit for tuning said generator oversaid range of operating frequencies, anda second balanced transmissionline comprising a pair of conductors individually directly coupled tothe other corresponding electrodes of said tubes and extending therefromin the same direction as said `llrst line, said second line having aneffective electrical length equal to an integral multiple of aquarter-Wave length at a predetermined frequency'above said range ofoperating frequencies and being electrically coupled to said rst linefor substantially all adjustments of said tuning l0 means fortransferring energy from said rst to said second line to sustainoscillations.

7. A balanced ultra-high-frequency oscillation generator tunable over apredetermined range of operating frequencies comprising, a. pair ofvacuum tubes individually having anode and control electrodes, a firstbalanced transmission line comprising a, pair of open conductorsindividually directly coupled to corresponding electrodes of saidtubes', adjustable tuning means included in said line and constitutingtherewith a frequency-adjusting circuit for tuning said generator oversaid range of operating frequencies, and a second balanced transmissionline comprising a pair of conductors individually directly coupled tothe other corresponding electrodes of said tubes and ex..

` tending therefrom in the same direction as said first line, saidsecond line having a resonant fre- `quency outside of said range ofoperating frequencies and including a loop portionl which iselectrically coupled to said rst line for substantially" all adjustmentsof said tuning means for transferring energy from said rst to saidsecond line to sustain oscillations.

8. A balanced ultra-high-frequency oscillation generator tunable over apredetermined range of operating frequencies comprising, a pair ofvacuum tubes individually having anode and control electrodes, a rstbalanced transmission line comprising a pair of open conductorsindividually directly coupled to corresponding electrodes of said tubes,adjustable short-circuiting means included in said line and constitutingtherewith a frequency-adjusting circuit for tuning said gener- 'atorover said range of operating frequencies, and

a second balanced transmission line comprising a pair of conductorsindividually directly coupled to the other corresponding electrodes ofsaid tubes and extending therefrom in the same direc-v generator tunableover a predetermined range of operating frequencies comprising, a pairof vacuum tubes individually having anode and control electrodes, afirst balanced transmission line comprising a pair of open'conductprsindividually directly coupled to corresponding electrodes of said tubes,adjustable tuning means including in said line and constitutingtherewith a frequency-adjusting circuit for tuning said generator oversaid range of operating frequencies, and a second balanced transmissionline comprising a pair of conductors individually directly coupled tothe other corresponding electrodes of said tubes and extending therefromin the same direction as said first line, said second line having aresonant frequency outside of said range of operating frequencies andincluding a reversed loop portion which is electrically coupled to saidrst line for substantially all adjustments of said tuning means fortransferring energy from said first to said second line to sustainoscillations.

10. A balanced ultra-highffrequency oscillation generator tunable over apredetermined range of operating frequencies comprising, a pair ofvacuum tubes individually having anode and control electrodes, a firstplurality of balanced transmission lines each comprising a pair of openconductors individually directly coupled to corresponding electrodes ofsaid tubes, adjustable tuning means included in said rst plurality oflines and constituting therewith a frequency-ad justing circuit fortuning said generator over said range of operating frequencies, and asecond plurality of balanced transmission lines corresponding in numberwith said first-named plurality and each extending therefrom in th`esame direction as a corresponding one of said first-named plurality oflines and each comprising a pair of conductors individually directlycoupled to the other corresponding electrodes of said tubes. said secondplurality of lines individually having a resonant frequency outside ofsaid range of operating frequencies and being electrically coupled to acorresponding one of said rst plurality of linesfor substantially alladjustments of said tuning means for transferring energy from said firstto said second plurality of lines to sustain oscillations.

11. A balanced ultra-high-frequency oscillation generator tunable over apredetermined range of operating frequencies comprising, a pair ofvacuum tubes individually having anode and control electrodes, a` ilrstpair of balanced transmission lines each comprising a pair of openconductors individually directly coupled to correspending electrodes ofsaid tubes, adjustable tunoperating frequencies, and a second pairofbalanced transmission lines each comprising a pair of conductorsindividually directly coupled to the other corresponding electrodes ofsaid tubes and each extending therefrom in the same direction asrespective ones of said rst-named pair ci lines, said second pair oflines individually having a resonant frequency outside of said range ofoperating frequencies and each being electrically coupled to acorresponding one of said first pair of lines for substantially alladjustments of said tuning means for transferring energy from said firstto said second pair of lines to sustain oscillations.

LEONARD R. MALLING.

REFERENCES CITED The following references are of record in the file ofthis patent:

Switzerland Mar. 16, 1937

